1. Technical Field
The presently disclosed technology relates to an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus of electrophotographic system commonly used for copying machine, printer, facsimile and the like, a photoreceptor having a photosensitive layer containing a photoconductive substance formed on a surface thereof is used. In this case, after an electric charge is given to a photoreceptor surface so as to uniformly charge therewith, an electrostatic latent image corresponding to image information is formed by various image forming processes. This electrostatic latent image is developed by a developer supplied from developing means, the developer containing a toner, so as to make a toner image. This toner image is directly transferred to a recording medium such as a paper, or once transferred to an intermediate transfer medium and then further transferred to the recording medium. And in order to fix the toner image transferred to the recording medium onto the recording medium, it is common to heat and pressurize the recording medium by fixing means of thermal fixing system using a developing roller or the like including heating means.
In an image forming apparatus of thermal fixing system, for instance, an intermediate transfer belt serving as the intermediate transfer medium is stretched by a heating roller. By so doing, the intermediate transfer belt and further the toner image formed on the intermediate transfer belt are heated, and transferred and fixed to the recording medium in a heated state (refer to, for instance, Japanese Unexamined Patent Publications JP-A 10-063121). An image forming apparatus disclosed in the JP-A 10-063121 is characterized by relatively small electric power consumption. However, this image forming apparatus has disadvantages of image deteriorations such that the toner image is not fixed and that offset of the toner image is caused, due to decrease of a temperature of the toner image when the toner image and the recording medium come into contact with each other at the time of transfer and fixation because the recording medium is not heated in this image forming apparatus.
In addition, an image forming apparatus is proposed in which the toner image on the intermediate transfer belt is heated as in the JP-A 10-063121 and moreover, the recording medium to which the toner image has not been transferred and fixed, is also heated so as to transfer and fix the toner image to the recording medium in the heated state (refer to, for instance, Japanese Unexamined Patent Publications JP-A 2004-151626).
In the image forming apparatus disclosed in the 2004-151626, fixing strength of the toner image to the recording medium is enhanced, but on the other hand, not only the toner image but also the recording medium are heated. Furthermore, heating means having a large heat capacity becomes necessary for heating the toner image and the recording medium and therefore, the electric power consumption becomes so larger as to occupy more than half of total electric power consumption in the image forming apparatus in the present circumstances. However, at the present time, saving energy is demanded as countermeasures against global warming. Since the image forming apparatus of electrophotographic system is commonly used, it is required also in the image forming apparatus of electrophotographic system to reduce the electric power consumption at the time when the toner is fixed to the recording medium Further, in the thermal fixing system, an interior portion of the apparatus has a high temperature by use of the heating means inside the apparatus as described above and therefore, it is necessary to enhance heat resistance of components, so that a material cost increases. Furthermore, in the thermal fixing system, a fixing operation cannot be performed until when a temperature of a fixing portion increases to the predetermined temperature. Accordingly, it tends to take a long time before the fixing operation can start, which time is a warm-up time in other words. Furthermore, the thermal fixing system has a problem that it takes a longer time for fixing a multicolored toner image to the recording image than fixing a monochromatic toner image. Consequently, it is desired to shorten a fixing time of the multicolored toner image.
In view of such desires, a wet fixing system is known which employs a fixer fluid containing water and a liquid dissolvable or dispersible in water, having an action for softening or swelling the toner. In this system, the toner image is fixed to the recording medium by attaching to the recording medium and pressurizing the toner image formed of the toner which has been made in a softened or swelled state due to application of the fixer fluid. The wet fixing system needs far smaller electric power consumption than that in the thermal fixing system. Accordingly, the wet fixing system is useful from a viewpoint of saving energy. Moreover, the fixing time of the multicolored toner image can be shorter than that in the thermal fixing system because a large heat quantity is not necessary. Consequently, various proposals have been made about further modification of the wet fixing system.
For instance, a fixing apparatus has been proposed in which the fixer fluid is applied from a fixer fluid spraying member having a plurality of pores to only a toner attached portion of the toner image carried on the intermediate transfer medium or recording medium, and then the applied fixer fluid is heated (refer to, for instance, Japanese Unexamined Patent Publications JP-A 2004-109747). That is to say, this fixing apparatus employs a system in which the toner image on the intermediate transfer medium or recording medium is heated after the fixer fluid is applied thereto. However, unfixed toner image is no more than an agglomeration of toner particles without physically or chemically binding force in a room temperature. Accordingly, when a liquid such as the fixer fluid is directly applied to the unfixed toner image, the toner particles are softened and/or swelled so that the toner particles easily become fluid or agglomerated. As a result, a blur is generated on an image edge after fixation and in addition, a halftone portion which should be even, have uneven density so that an image of high quality cannot be obtained. It goes without saying that this state cannot be brought back into good condition even when the toner image is heated after the toner particles become fluid or agglomerated.
Further, in the fixing apparatus disclosed in the JP-A 2004-109747, in a case where the toner image is carried by the intermediate transfer medium, the intermediate transfer belt serving as the intermediate transfer medium is treated with water-shedding processes such as fluorine treatment. Accordingly, even when the fixer fluid is applied to the intermediate transfer belt, the fixer fluid exists only on a toner portion (an image portion) while the fixer fluid does not stagnate on a portion between the toner images, where the toner does not exist (a non-image portion). However, when the fixer fluid is thus applied locally to the recording medium, the image portion expands and contracts while the non-image portion does not expand or contract, and therefore it is not possible to prevent wrinkles from being generated around the image portion. Particularly, in a case where a recording paper manufactured by combining paper fabric dispersed in water is used as the recording medium, this tendency is prominent. As a matter of course, when the fixer fluid of the minimum amount for swelling the toner is applied, such troubles are not caused. However, it is difficult to exactly measure the minimum amount because the minimum amount is extremely small. Further, when the fixer fluid is applied to only the toner attached portion, the toner attached by fog or the like on the non-image portion around the toner attached portion remains on the recording medium without being fixed so that the toner may mess hands, clothes or the like.
Note that in the image forming apparatus in the JP-A 10-063121, it is conceivable that the fixer fluid is applied to the toner image when the toner image on the transfer belt in the heated state is transferred and fixed to the recording medium which has not been heated. However, in order to enhance adherence between the toner image and the recording medium and adherence among the toner particles in a case where heat is not supplied at the time of transfer and fixation, and no other particular measures are taken as in the case of the JP-A 10-063121, a large amount of the fixer fluid becomes necessary. When the large amount of the fixer fluid is used, it is not possible to prevent wrinkles, curls and the like from causing on the recording medium. In addition, it becomes necessary to replenish the fixer fluid with frequency, with the result that maintenance is deteriorated. Alternatively, a high-capacity fixer fluid storing tank becomes necessary, with the result that the apparatus cannot be small in size.